Superconductive materials containing thallium, barium, calcium, and copper cations are well known. Many of these materials have a relatively high onset temperature ("Tc"), allowing them to be used at higher temperatures than many of the older superconductive compositions.
The thallium family of superconductors has the general formula Tl.sub.a Ba.sub.2 Ca.sub.b Cu.sub.b+1 O.sub.c, wherein a is from about 1 to about 2, b is from 0 to 2, and c is equal to 2(b+1)+4. As is known to those skilled in the art, in such formula barium may be partially substituted by strontium up to about a mole ratio of about 1:1 of strontium to barium. Similarly, in such formula the thallium may be partially substituted by bismuth up to about a mole ratio of about 1:1 of bismuth to thallium. It is preferred, however, to have an embodiment in which there is no substitution of the thallium by bismuth and no substitution of the barium by strontium.
U.S. Pat. No. 5,039,653 of Jackson et al. discloses a process for growing a superconductor material in a fluxed melt. Although no specific examples of the use of this process to produce a thallium superconductor are disclosed in the Jackson patent, it is disclosed (at column 2 of the patent) that ". . . the invention is applicable to thallium-barium calcium cuprate superconductor material . . . ." The disclosure of this patent is hereby incorporated by reference into this specification.
In the examples of the Jackson et al. patent, which relate to the production of bismuth superconductor, a strontium carbonate and a bismuth oxide starting material are used. These materials, together with copper oxide and calcium hydroxide, were reacted, the resulting body was ground into a powder, and the powder was then mixed with sodium chloride and then heat-treated. Large crystals (on the order of at least about 2 millimeters diameter) in the form of platelets are formed in the process of Jackson et al.; these large crystals are taught to be useful for SQUID applications. Although these crystals may be useful for certain purposes, they are not desirable for use in spin on films and tape casting.
When the experiment described in the examples of the Jackson patent is modified to replace strontium carbonate with barium carbonate and bismuth oxide with thallium oxide, a superconductive material is formed. However, this material has a relatively low onset temperature.
It is an object of this invention to provide a process for the preparation of a thallium superconductor with a relatively high onset temperature.
It is another object of this invention to provide a process for the preparation of thallium superconductor which will require substantially less energy than the process of the Jackson patent.
It is yet another object of this invention to provide a process for the preparation of thallium superconductor wherein substantially all of the particles in such superconductor are smaller than about 20 microns.
It is yet another object of this invention to provide a process of thallium superconductor material which may be screen printed onto an alumina substrate to produce a circuit board with improved critical current density properties.